1. Field of the Invention
The present invention relates to a controller for a permanent magnet synchronous motor usable for an electric vehicle and the like. More specifically, the present invention relates to a controller for performing a flux-weakening control or a maximum efficiency control by supplying a direct axis (D-axis) current value to such a synchronous motor in order to maximize the efficiency of the motor.
2. Description of the Related Art
In the proceeding of the Institute of Electrical Engineers of Japan, a Publication of Electric Applications Society IEA-92-30 (Document 1), a flux-weakening control is proposed to be performed by calculating a command value i.sub.d * of a field current in accordance with the following Equation 1. EQU i.sub.d *=-i.sub.dz *(.omega.-.omega..sub.base)/(.omega..sub.max-107 .sub.base) (1)
where, .omega..sub.base indicates a rotation speed of the base, .omega..sub.max indicates a maximum rotation speed, i.sub.dm indicates a field current at the maximum rotation number .omega..sub.max.
On the other hand, in the proceeding of the Institute of Electrical Engineers of Japan, No. 74, pp. 310 to 315 (Document 2), a flux-weakening control is proposed to be performed by calculating a command value i.sub.d * of a field current using a target rotation number, a D-axis winding reluctance, a quadrature axis (Q-axis) winding reluctance, a stator winding resistance, an unloaded induced voltage at a unit speed, and the like.
U.S. Pat. Nos. 4,361,791 and 4,649,331 disclose methods for supplying the field current i.sub.d to a motor in accordance with the state of the motor at real time by detecting an error (or the degree of the realization) between the value of a motor torque or a current actually measured by a controller at real time and a command value so as to feedback the error.
According to conventional methods such as those disclosed in Documents 1 and 2, in order to perform a flux-weakening control in a high-speed rotation region, the field current i.sub.d is supplied in accordance with the target rotation number .omega. using Equation 1.
However, when a motor is actually operated, the constant of the motor is varied in accordance with the variation of the resistance value under various operational conditions and the passage of time. For example, if the temperature in the vicinity of an ambient temperature increases by 70 degrees, then the electric resistance of the motor increases by about 30%. Accordingly, the field current i.sub.d obtained by Equation 1 used by these conventional methods is not always an optimal field current i.sub.d.
Therefore, in order to output a high torque at a high-speed rotation, it has conventionally been required to supply an excessive amount of field current i.sub.d in consideration of the variation of these characteristics. For example, when the inductance is 4 mH, the stator frequency is 133 Hz, and the applied voltage is 80 V, the field current i.sub.d required for realizing an inverse electromotive voltage of 80 V and a torque current of iqlA is 1.95 A at the resistance of 1.OMEGA., or 2.63 A at the resistance of 1.3 .OMEGA..
Consequently, the excessive amount of the field current i.sub.d increase the copper loss, so that the efficiency is degraded.
On the other hand, according to the methods for supplying the field current i.sub.d to be obtained by the feedback control as disclosed in Japanese Laid-Open Patent Publications Nos. 57-196896 and 62-7396, the design rule or the control operation to be insufficient for realizing a stable operation to be executed easily. That is to say, the methods have the following problems.
1. According to these methods, very large variation of the characteristics can not be processed easily.
2. These method fail to provide a method for improving the response characteristics.
3. It is impossible to supply a field current optimized for the dynamic operation and the regenerative operation of the motor, respectively.
4. Other kinds of method for improving the efficiency excluding the calculation of the field current i.sub.d by using the feedback control are not mentioned.
5. In the case where the voltage of a battery is lowered and the terminal voltage to be supplied to the synchronous motor is also lowered, the amount of the D-axis current becomes excessive with respect to that of the total current, so that the efficiency is degraded.
6. Only one reference value can not allow for satisfying the maximization of the efficiency and the maximization of the torque at the same time.
7. Since it is impossible to control the D-axis current and the Q-axis current separately, the response characteristics of one of the currents can not help being sacrificed.
8. In the case where a failure is generated in the circuit or in the system, the processing to be performed is not definitely described.